1. Field of the Invention
The present invention relates to a Hall device biasing circuit for applying a bias voltage to each of at least two Hall devices, and a magnetism detection circuit including the same.
2. Description of the Related Art
A Hall device is used for magnetism detection in a wide range of fields including motor driving circuits, pickup control of optical disks and the like, focusing control of cameras, and TPS mounted on automobiles and the like.
FIG. 6 shows an operating principle of a Hall device. When a magnetic field having magnetic flux density B is applied to a Hall device 41, the Hall device 41 outputs a voltage VH in proportion to the applied magnetic flux density B.
Systems for driving a plurality of Hall devices are classified into a constant current driving system for supplying a constant current to drive the Hall devices (in which the Hall devices are connected in series) and a constant voltage driving system for applying a constant voltage to drive the Hall devices (in which the Hall devices are connected in parallel).
FIG. 7 shows a practical example of a constant voltage driving system for driving a plurality of Hall devices 41a through 41c connected in parallel. FIG. 8 shows a practical example of a constant current driving system for driving a plurality of Hall devices 41a through 41c connected in series.
The constant voltage driving system shown in FIG. 7 operates in the following manner. A constant voltage is applied to each of the Hall devices 41a, 41b, and 41c connected in parallel from a constant voltage source 42. Then, signal output terminals H1+ and H1xe2x88x92 of the Hall device 41a, signal output terminals H2+ and H2xe2x88x92 the Hall device 41b, and signal output terminals H3+ and H3xe2x88x92 of the Hall device 41c each output a voltage in proportion to a magnetic flux density applied to each of the Hall devices 41a, 41b and 41c. 
According to this system, the Hall devices 41a, 41b and 41c are each driven at a constant voltage.
The constant current driving system shown in FIG. 8 for driving the Hall devices 41a, 41b, and 41c connected in series is disclosed in, for example, Japanese Laid-Open Publication No. 9-65682. A voltage from a motor driving circuit 43 is applied to the Hall devices 41a, 41b, and 41c, and the Hall devices 41a through 41c are each driven by a constant current. Accordingly, even when the number of Hall devices is increased or decreased, the driving current of the entire driving circuit does not change much.
An output voltage VH of the Hall device driven by the constant current driving system is represented by expression (1), and an output voltage VH of the Hall device driven by the constant voltage driving system is represented by expression (2).
VH=(RH/d)xc2x7Icxc2x7Bxe2x80x83xe2x80x83(1)
VH=xcexcHxc2x7(W/L)xc2x7Vinxc2x7Bxe2x80x83xe2x80x83(2)
In the expressions (1) and (2), B represents the magnetic flux density applied to the Hall device, d represents the thickness of a magnetic field sensing portion in the Hall device (thickness of the Hall device), and W and L respectively represent the width and length of the magnetic field sensing portion in the Hall device with respect to the driving voltage. Ic represents a driving current in the constant current driving system, and Vin is a driving voltage in the constant voltage driving system. In expression (1), RH represents a Hall coefficient of the Hall device and is represented by RH=1/(exc2x7n), where e is a charge amount of electrons, and n is a carrier concentration of the Hall device. In expression (2), xcexcH is an electron mobility of a semiconductor in the Hall device.
It is now assumed that the magnetic flux density B is constant and the size of the magnetic field sensing portion of the Hall device is constant. By the constant current driving system, since the driving current Ic is constant, the output voltage VH of the Hall device is in proportion to the Hall coefficient RH based on expression (1). By the constant voltage driving system, since the driving voltage Vin is constant, the output voltage VH of the Hall device is in proportion to the electron mobility xcexcH based on expression (2). Generally, it is known that the Hall coefficient RH is high in temperature dependency and that the electron mobility xcexcH is low in temperature dependency.
The constant voltage driving system shown in FIG. 7 provides better temperature characteristics than the constant current driving system, and thus the Hall devices 41a through 41c in FIG. 7 each output a voltage which is stable against a change in the ambient temperature. However, the constant voltage driving system requires a driving current in proportion to the number of Hall devices used, and thus has a disadvantage in that when a great number of Hall devices are used, the current consumption cannot be suppressed. This can be a very serious problem in a circuit having an especially large number of Hall devices due to the significant increase in the current consumption.
By the constant current driving system shown in FIG. 8, a driving current provided by the motor driving circuit 43 as a power source is used sequentially for the Hall devices 41a through 41c which are connected in series. Accordingly, even when the number of Hall devices is increased, the amount of the driving current passing through the Hall devices is not increased. However, in such a case, the problem that the temperature dependency is raised as described above.
According to one aspect of the invention, a Hall device biasing circuit includes a plurality of terminals for applying a bias voltage to a plurality of Hall devices connected in series, respectively.
In one embodiment of the invention, the Hall device biasing circuit further includes a constant voltage supply section for supplying a constant bias voltage to each of the plurality of terminals.
In one embodiment of the invention, the constant voltage supply section includes a constant voltage supply circuit in correspondence to the plurality of Hall devices.
In one embodiment of the invention, the constant voltage supply section includes: a constant voltage supply circuit, at least one current path through which a bias correction current flows from one of the plurality of terminals to another of the plurality of terminals, and a correction current supply section for selecting one of the at least one current path based on a current amount therein and adjusting the current amount in the selected current path to supply the bias correction current.
In one embodiment of the invention, the correction current supply section includes: a constant voltage generation circuit, and a comparison section connected to the at least one current path for selecting one of the at least one current path based on a current amount therein and adjusting the current amount in the selected current path based on a current amount flowing through the at least one current path and a current amount generated in the constant voltage generation circuit.
In one embodiment of the invention, the constant voltage supply section uses a supply voltage outside the Hall device biasing circuit.
In one embodiment of the invention, the constant voltage supply section includes: at least one current path through which a bias correction current flows from one of the plurality of terminals to another of the plurality of terminals, and a correction current supply section for selecting one of the at least one current path based on a current amount therein and adjusting the current amount in the selected current path to supply the bias correction current.
In one embodiment of the invention, the correction current supply section includes: a reference voltage source, and a comparison section connected to the at least one current path for selecting one of the at least one current path based on a current amount therein and adjusting the current amount in the selected current path based on a current amount flowing through one of the plurality of terminals and a current amount generated in the reference voltage source.
In one embodiment of the invention, the comparison section includes a comparator circuit.
In one embodiment of the invention, a positive terminal of the comparator circuit is connected to a terminal of one of the plurality of terminals, and the negative terminal of the comparator circuit is supplied with a reference voltage generated in the reference voltage source.
In one embodiment of the invention, the comparator circuit includes a buffer amplifier.
In one embodiment of the invention, the at least one current path includes a switching device.
In one embodiment of the invention, the at least one current path includes an OR circuit and an inverter.
In one embodiment of the invention, the correction current supply section includes: a plurality of reference voltage sources respectively provided in correspondence to the plurality of Hall devices, and a plurality of comparison sections each connected to each of the at least one current path for selecting one of the at least one current path based on a current amount therein and adjusting the current amount in the selected current path based on a current amount flowing through one of the plurality of terminals and a current amount generated in one of the plurality of reference voltage sources corresponding to the one of the plurality of terminals.
In one embodiment of the invention, the correction current supply section further includes a resistor between the reference voltage source and the comparison section for dividing a resistance of the reference voltage source.
In one embodiment of the invention, a former-stage terminal with respect to one of the plurality of terminals corresponding to the buffer amplifier is connected to a positive power supply of the buffer amplifier, and a latter-stage terminal with respect to the one of the plurality of terminals corresponding to the buffer amplifier is connected to a negative power supply of the buffer amplifier.
According to another aspect of the invention, a magnetism detection circuit includes a plurality of Hall devices connected in series; and a Hall device biasing circuit including at least a plurality of terminals corresponding to the plurality of Hall devices for supplying a constant bias voltage to each of the plurality of Hall devices respectively from the plurality of terminals.
In one embodiment of the invention, the Hali device biasing circuit further includes a constant voltage supply section.
In one embodiment of the invention, one end of the series of plurality of Hall devices is grounded.
Thus, the invention described herein makes possible the advantages of providing a Hall device biasing circuit for driving each of a plurality of Hall devices so as to maintain the satisfactory temperature dependency without increasing the driving current even when the number of Hall devices is increased, and a magnetism detection circuit including such a Hall device biasing circuit.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.